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As show on the figure, this relationship has many applications, especially to Virginia's cropping systems. In most full-season production systems (planted in May and early June), the soybean crop will usually obtain the minimum LAI level of 4.0. This is because this system has adequate time to develop its leaf area. Only on unproductive soils and under situations of drought would a full-season soybean yields be reduced by lack of leaf area. On the other hand, double-crop plantings, especially those following wheat, will in many cases not obtain the necessary leaf area to maximize yield potential. This is due to the relative short vegetative growth period before flowering. This becomes further aggravated when one chooses early-maturing soybean varieties.
Therefore, when planting double- crop, one should choose the latest maturing variety possible that will mature before frost in order to allow adequate leaf area buildup. With full-season plantings, maturity group has much less of an impact on leaf area.
The following figure describes these relationships. In addition, soybeans with an LAI of less than 4.0 will most likely lose yield to insect defoliators and herbicide injury.
Another factor that has a large influence on soybean leaf area is soil type. The main reason that certain soils do not yield as well as others is water-holding capacity. While plant-available water is the most important factor contributing to yield, the inability of the less productive soils to develop adequate leaf area also limits yield. When LAI is limiting, such as a double cropping system, and if no severe drought occurs during seed fill, higher LAIs will result in higher yields. Therefore, if LAI can be increased via narrowing row spacing or increasing plant population on these moisture-limiting soils, then higher yields are possible.
If the yield is directed related to LAI under situations in which adequate LAI cannot be obtained such as late plantings or when grown on relatively unproductive soils, then the goal should be to utilize production practices that can increase and maintain soybean LAI under these circumstances. Certain cultural practices such as narrowing row spacing, increasing plant populations, and using longer maturing varieties will increase LAI. Therefore, one should utilize such practices in situations in which LAI is expected to be lower. Additionally, one should make efforts to protect the crop from LAI-reducing insect defoliators or injurious herbicides if LAI is low.
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No tillage is probably the single most important practice that producers can implement to reduce expenses and labor, maintain soil quality, and minimize off-site movement of soil and nutrients. In Virginia, nearly all of our double-crop acres and almost half of our full-season acres are not tilled. In my opinion, we should strive for planting 100% of all soybean acres without tillage. However, in order to adequately develop the needed canopy cover, we have to obtain a nearly perfect stand. With no-till, we can experience problems with this unless our planting equipment is set properly.
I feel that there is a mistaken belief that in order to convert a planter or drill into a no-till piece of equipment, one must put coulters in front of the planting units. In the early days of lightweight planters or drills with runners or small-diameter disk seed furrow openers, this was exactly the case because this equipment could not penetrate untilled soil or close the seed furrow. Although we called coulters a no-till attachment, they were actually overcoming these shortcomings by tilling the soil in a narrow strip. Although, I'm not opposed to coulters and I like to see their use in sandy soils (coulters take the wear and tear off of the disk openers), they are not the attachment that converts an ordinary planter or drill into a no-till piece of equipment. The discussion below is the reason why I make these statements and discusses the principles of no tilling.
At last year's Virginia Corn and Soybean Conference, Paul Jasa, Extension Engineer for the University of Nebraska listed the following requirements that are necessary to obtain a good stand in no-tillage systems.
Planters and drills must:
Keeping these three things in mind, a producer can evaluate the strengths or weaknesses of any piece of planting equipment and make adjustments necessary for no-till conditions. Fortunately, currently available planters and drills can be used for no-till with little, if any modifications.
Cutting and Handling Residue. One of the problems with older planters was the size of the disk seed-furrow openers. In order to cut through residue, these disk openers must contact the soil at an angle of approximately 45 degrees (This holds true for coulters mounted in front of the planting unit as well). If the angle of contact is greater than this, the coulter is too deep and the disk basically pushes the residue. This will occur if the coulter is too small. In contrast, if the angle of contact is less than 45 degrees, then youčre basically running over the residue. Both situations will cause hairpinning of the residue. However today's planters will usually have a large enough disk opener to avoid these problems. But, keep this point in mind if using coulters up front.
If the two furrow-opening disk blades are mounted side-by-side, they should have about two inches of blade contact on the leading side and should work together as a single cutting edge; no gaps should be visible between the two edges. If a gap exits, the individual disks can be adjusted inward as they wear by removing spacer washers from behind them. On staggered disk models, the rear disk should be tucked in behind the leading disk, but not wedged tightly together, or the rear disk will literally cut the front one off. Still, one should only be able to fit the width of a business card between the two disks. On some drills, the manufacturer has eliminated the second disk and mounted a larger diameter single disk on a slight angle, providing minimal disturbance and eliminating the above issues. If opening disks are properly adjusted, they become a much sharper residue-cutting device than any coulter mounted up front.
Although these disks can easily cut residue if properly adjusted, sandy soils will easily wear them out; therefore, a coulter used up front will take the brunt of these abrasive conditions, increase disk opener life, and minimize time devoted to adjustment.
If one feels that there is too much mulch to cut through or if they feel that the crop would benefit from removing the residue (i.e., cold soils), then a variety of disk row cleaners can be used to move residue out of the row. If using these row cleaners, be sure to set them to move only residue and not soil, or other problems will be created.
Penetrating the Soil to the Desired Seeding Depth. The second criteria for obtaining good stands are to get the seed down through the residue and into the soil. Be aware that the getting the residue cut must precede this second step. Unless the residue if cut, the seed cannot be placed through it. To insure this step, a second critical no-till attachment (the first is the furrow-opening coulters) is needed - heavy-duty downpressure springs. These are needed to transfer weight from the planter toolbar to the individual row units. Unlike coulters in front of the planting unit, downpressure springs are a must. Most don't realize that it takes 400 to 500 pounds of force per unit to penetrate the soil. Heavy-duty downpressure springs can accomplish this. There must also be sufficient weight on the units to keep the depth gauge wheels in firm contact with the ground to control planting depth. If the gauge wheels are loose, tighten the springs. If the gauge wheels are tight, but this force is pulling the planter drive wheel off the ground, then add more weight to the planter.
Drills present a unique problem with the above recommendations because of the number of row units per drill width. For example a six-row 30-inch planter (15 feet wide) may require up to 3,000 lbs of weight in order to penetrate the soil and get the proper angle on the disk openers to cut the residue (6 rows x 500 lb./row). But a 15-foot 7.5-inch row drill can require 12,000 pounds of weight (24 rows x 500 lb./row). On drills with a single disk opener, less weight may be needed because these drills have only half the number of disks to penetrate the soil. But, never underestimate the amount of weight needed.
Establishing Proper Soil-to-Seed Contact. The final step actually overlaps with the previous step in a way. Sufficient weight must remain on the press wheels to ensure firming of the seed into the soil. However, take care not to over pack the soil. Setting the closing wheels wrong can result in compaction and the new, young seedling will have a hard time establishing its new roots. Most planters and drills allow adjustment of the closing wheels separate from the disk openers. Only apply enough pressure to close the seed-V, never more. Planters or drills that rely on the closing wheel for depth control creates problems in no-till and should be avoided if possible.
The key is to evaluate soil-to-seed contact, not contact at the top of the seed-V or contact below the seed. Therefore, the slot does not necessarily have to be closed. A problem with many planters is the angle of the two closing wheels. Many are set at an angle that provides maximum soil-to-seed contact at 2 inches; about 0.5 to 1.0 inch deeper than I like to see soybean planted. In order to determine where maximum soil-to-seed contact will occur, draw two imaginary lines from the closing wheels using the same angle of the wheels into the soil. The point at which these lines intersect will be the depth of maximum pressure. If too deep or too shallow, then adjust.
Pre-plant Field Check. Before putting any seed into the planter, take it out to a typical field that will be planted. Level the planter, making sure that the toolbar is at the proper height and leveled front to back. Never set the planter nose down. If you have problems closing the seed-V, then one may want to adjust the planter so that it is slightly "tail" down. By leveling the planter, this allows the full-rang of movement on the parallel links on the row unit, helps keep the planter on the row, and aids in soil-to-seed contact. If there are any ridges in the field from last year's cultivation or a subsoiler, the make sure that the carrying wheels are exactly centered between the rows and are carrying some weight. The part of the planter carrying the most weight will always move to the lowest center of gravity; therefore, if the row units are carrying the most weight, then they will move to the furrow. If this is a problem, add weight to the planter.
After leveling, blind plant and check the performance by evaluating the three functions mentioned earlier. Make any adjustments. Buy attachments if needed, but only after evaluating the problem that you have and determining how an attachment will help.
With proper adjustments, most drills and planters will function as a no-till piece of equipment. Time spent understanding the principles of setting the equipment should avoid problems and prevent purchasing unneeded attachments.
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Poor germination, emergence, and sometimes death of soybean seedlings are often the result of seed and seedling diseases caused by an assortment of fungi and bacteria that reside in the soil, seed, or crop residues. Seed most frequently become infected with the fungi or bacteria while the soybean plant is maturing in the fall, especially under conditions of warm, moisture weather. Many times, poor seed quality is the result of delayed harvest. Improper drying and storing may also lower seed quality. However, if one selects seed with 85% or better in warm germination tests (listed on the bag), seed treatments are not usually needed unless planting occurs before the soil has warmed to an average of 65°F. If planting into cold soils (late April or early May), one may want to consider a fungicide seed treatment, especially if the weather forecasts do not indicate that soils will warm. If germination is below 85% and one is planting into cold soils, then a seed treatment should definitely be considered. However if germination is less than 75%, a fungicide seed treatment may help, but is not the solution to the problem. In this case, one should either re-clean the seed and re-test for germination or find another source of seed.
If planting into cold soils and the weather forecasts continued cool conditions, especially if germination is less than 85%, one should consider a chemical seed treatment containing one or more fungicides. Theses fungicides can be applied as seed dressings by commercial applicators that use specialized equipment to achieve a precision rate and uniform coverage or by the producer via hopper box or on-farm seed treatment equipment. An effective fungicide seed treatment should contain either captan or thiram as the base. Carboxin, PCNB, or metalaxyl may be added as supplements. Captan and PCNB can reduce populations of nitrogen-fixing bacteria; therefore do not use these fungicides in combination with soybean inoculant. Broad-spectrum treatments approved for on-farm use generally include a mixture of thiram, carboxin, and/or metalaxyl.
In summary, I would recommend seed treatments if lower quality seed is being used and soil temperature is averaging less than 65°F. Usually, there is a risk of cool soils in early May, but temperatures usually warm up during the middle of the month. However, as in 2000, an extended cold snap can occur in late May. During that year, we planted soybean no-till into relatively cool soils, but greater than 65°F. A cold rain and low temperatures followed this for the next 5 days. These soybean took two weeks to emerge. Fortunately, all seed were treated with fungicide and a good stand was established. So, keep your eye on the weather forecast before planting soybean into cold soils. A fungicide seed treatment may be of some value in this case.
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For full-season systems, row spacings of 20 inches or less and final plant populations of 100,000 to 140,000 plants per acre are recommended. Note that these are final plant populations, not seeding rates; the amount of seed planted per acre should be adjusted for expected percent emergence.
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Sincerely,
David L. Holshouser